1. Field of the Invention
The present invention relates in general to a compensating device for improving scanning luminance. More specifically, it relates to a compensating device using crutch-shaped light tubes for improving the luminance at two ends of a scanning central line. In the present invention, at least two crutch-shaped light tubes are used to form a light-radiating device, thereby improving the light intensity and uniformity.
2. Description of the Related Art
FIG. 1 shows an image-scanning device of a first example of the prior art, which comprises a light-radiating device 14 and a focusing-and-imaging device 16. The light-radiating device 14 may be a cold-cathode tube having a straight line tube 14 and two connecting heads 144 at its ends. The focusing-and-imaging device 16 may consist of a lens set.
The light-radiating device 14 moves with respect to an object 10 which is, for example, a piece of paper or a picture, along a scanning path as indicated by arrow A. The object 10 reflects the light emitted by the light-radiating device 14, as indicated by arrows B. The focusing-and-imaging device 16 then focuses an image of the reflected light onto the image-receiving device 12 as indicated by arrow C. For instance, the image-receiving device 12 may be a charge-coupled device (CCD).
However, the object light projected onto the image-receiving device 12 is not uniform. The light corresponding to the middle portion of the object 10 is reflected more than that corresponding to both edges of the object 10, thereby causing image distortion. Increasing the length of the straight-line tube 142 cannot effectively improve the problem and merely increases the bulk of the image-scanning device.
Additionally, the image-scanning device of the first example has only one straight light tube 142, and therefore cannot generate enough light for rapid scanning.
FIG. 2 shows an image-scanning device of a second example of the prior art, in which two light-radiating devices 14 are provided to generate sufficient light for rapid scanning. However, the intensity of the light projected onto the image-receiving device 12 is still nonuniform, therefore resulting in image distortion (as mentioned in the case of using a single tube). Furthermore, the illumination at the ends of the scanning central line is not improved, and the problem of nonuniform light intensity reflected at the edges of the object 10 is not solved.
In general, the light emitted from a straight-line light tube is nonuniform. The light intensity emitted from the center of a straight-line tube is stronger than those from the two ends of the straight-line tube, and thus the luminance at the center of the scanning central line is greater than that at two ends of the scanning central line. Consequently, the image received by the charge coupled device has distortion, thereby reducing the signal accuracy and scanning quality. When using two straight-line light tubes, the contrast (difference) of luminance between the edges and the center of the scanned object increase dramatically, thereby further deteriorating the scanning quality. As a general solution, the length of the straight-line light tube is extended to overcome the above problem, and the areas of weak luminance at both ends of the straight-line light tube are not used for scanning. In this manner, only the portion of the straight-light tube with uniform luminance is used for scanning. However, the size of the scanner will necessarily increase.